The use of polymers in separation processes is well known in a variety of industries from water treatment to industrial gas separations. The polymers generally accomplish separation through a solution-diffusion mechanism when utilized as a non-porous material, and through molecular sieving or Knudson diffusion when utilized as porous materials. Additionally, polymers have been fabricated for a combination of porous and non-porous behaviors with a morphology characterized as asymmetric. An asymmetric polymer material is generally characterized by a dense thin film providing high selectivity, and supported by a porous polymeric network having high permeability. In polymer membranes utilized for gas separations, the asymmetric morphology is advantageous in that it allows a component selectively passed through the dense layer to proceed through the lower structure with minimized resistance.
In polymer membranes utilizing an asymmetric morphology, it is important that the preferred component diffuse through the dense layer at a sufficient flow rate, which makes if desirable to keep the thickness of the dense layer as small as possible. Additionally, typically the dense film cannot have a porous surface in order to preserve the inherent selectivity of the dense film in gas separation operations. These two requirements present significant difficulty in connection with the fabrication of the asymmetric morphologies, since dense films of a thickness of 1 micron or less can rarely be manufactured without flaws. Fabrication of asymmetric morphologies having a thin and defect-free dense film has been an area of significant effort. These methods frequently employ a dry-wet spinning process using a dope containing swelling agents in order to provide for formation of the dense skin in the dry air gap, followed by precipitation of a porous polymer matrix in the coagulation bath. The swelling agent is miscible with the non-solvent in the coagulation bath in order to produce the dense film supported by the porous polymer network. The See e.g., U.S. Pat. No. 4,673,418 to Peinemann, issued Jun. 16, 1987, among others. In these fibers, where the selectivity of the dense layer provides for gas separation, the exceedingly small dimensions of gas molecules make the presence of even Angstrom size pores in the separation layer unacceptable, and the emphasis is on formation of the defect-free dense film.
It would be advantageous to provide a fiber having an asymmetric morphology which is less sensitive to the presence of defects within the dense film for satisfactory separation operations. It would be further advantageous if a well known technology such as dry-wet spinning could be utilized to fabricate the fiber.
In order to combat the requirement for defect-free dense films, polymer fiber structures have also been utilized as supporting contactors between a gaseous mixture and a supported carrier or sorbent. Typically a sorbent or transport medium is supported by a hollow fiber structure, in order to take advantage of the contact area per unit volume ratio afforded by the hollow fiber geometry. The sorbent-supporting fibers may be fabricated with an asymmetric morphology having a dense film, or may utilize an essentially uniform porosity through the fiber in order that a multi-component mixture permeates the fiber surface and encounters the supported sorbent. In the latter case, unlike the purely polymeric fibers, the sorbent-supporting polymer mainly provides structural support, since the composite material relies on the supported material to provide selectivity.
In the sorbent-supporting fibers, the supported sorbents may be a solid or liquid sorbent. Solids sorbents typically produce hybrid materials with the solid sorbents entrapped within the pores of a porous polymer matrix. See e.g., U.S. patent application Ser. No. 12/163,140 filed by Lively, filed Jun. 27, 2008, published Jan. 29, 2009. These materials utilize a “sieve in a cage” morphology where discontinuous physical contact exists between the dispersed solid particles and the polymer matrix. The discontinuous physical contact holds the solid particle in place while allowing for facile gas bulk diffusion throughout the interconnected pore structure of the fiber, in order to avoid occluding access to the pores of the sorbents. The solid particle sorbents are selected based on sorption capacities for a given chemical species that vary with respect to temperature, and separation of the given chemical species occurs through sorption/desorption using a temperature swing process. These systems also commonly detail a hollow fiber geometry, so that necessary heat transfers may occur from or to a fluid flowing through the interior of the hollow fiber.
These hybrid membranes utilizing solid sorbents often fall short of predicted separation performances due to polymer rigidization and poor polymer/sorbent contact arising as a result of fabrication methodologies. Generally, the solid supporting fibers are fabricated by dispersing the solids in a polymer dope and extruding the mixture in a spinning process, so that the porous polymer network forms through solvent separation in the presence of the solids to be supported. The nature of this arrangement leads to localized stresses as the polymer contracts around the solid sorbent in the membrane. Compressive stresses can lead to rigidification of the polymer matrix around the sieve, leading to lower diffusivity in that region, while tensile stresses can lead to delamination of the polymer and the solid sorbent, resulting in gaps at the interface. See e.g., Das et al., “Gas-Transport-Property Performance of Hybrid Carbon Molecular Sieve-Polymer Materials”, Ind. Eng. Chem. Res. 49 (2010).
When liquids are utilized to fill the pores of the polymer supporting matrix, the hybrid material typically functions as a membrane, with separation of a chemical species from a mixture occurring simultaneously with removal of the chemical species from the membrane at another membrane boundary. In hollow fiber geometries, typically the exterior of the hollow fiber is exposed to the gaseous flow containing the chemical species to be separated, and separated species is removed through the interior of the hollow fiber. In many cases, the liquid contained within the fiber is a reactive solution, and the separation occurs by facilitated transport. The facilitated transport process is based on absorption of the chemical species by the liquid at pores on the exterior surface, reaction of the species with a chemical component present in the liquid, transport of the resulting chemical complex by diffusion through the liquid-filled pore network, decomplexation to form the original chemical species, and release into the hollow interior of the fiber. This approach increases the flux through the membrane and enhances selectivity; however, the finished products generally suffer from membrane instability arising due to gradual loss of the liquid membrane. The principal mechanisms leading to this loss may include the solubility of the carrier and its diluent in the feed and strip fluids in the case of liquid/liquid separations, volatilization of the carrier or its diluent in the case of gas/gas separations, or capillary displacement as a result of an osmotic pressure differential between the two sides of the membrane. Various methodologies have been employed to combat the liquid loss. See e.g., U.S. Pat. No. 6,086,769 to Kilambi et al., issued on Jul. 11, 2000, among others. Additionally, fabrication of the liquid-filled polymers has been limited to methodologies where fiber fabrication occurs as a separate spinning step, and filling of the porous polymeric network occurs by soaking the finished fiber in the liquid to be supported for a sufficient time.
It would be advantageous to provide a fiber containing an immobilized liquid within the pores of a porous polymeric network which could be produced without reliance on the two separate fabrication steps of spinning and soaking. Simpler fabrication could provide for utilization of liquid supporting fibers on a wider scale. Additionally, the immobilization of a liquid in the simpler fabrication would avoid the issues associated with the reduced performance of solid supporting fibers, such as reduced diffusivity and delamination.
Ionic liquids have been utilized as a solvent for gaseous separation in supported ionic liquid membranes (SILM) utilizing porous polymer networks. See e.g., U.S. Pat. No. 6,579,343 issued to Brennecke et al., issued Jun. 17, 2003. However, the ionic liquids are typically incorporated into the porous polymer network by a soaking process, which requires an interconnected polymer network having sufficient fluid communication with the external environment in order for a successful soaking step. The substantial transmembrane pressure combined with fluid communication to the environment can lead to relatively rapid ionic liquid loss and ineffectiveness of the hybrid material. It would be advantageous to provide a process whereby an ionic liquid could be immobilized in a porous polymer network without reliance on the soaking step and without the necessity for fluid communication between the porous network and the external environment, in order to mitigate ionic liquid losses during operation.
Accordingly, it is an object of this disclosure to provide a method for production of a fabricated fiber comprised of a porous polymer network and an immobilized ionic liquid within the pores of the network.
Further, it is an object of this disclosure to provide a method for production of a fabricated fiber comprised of a porous polymer network and an immobilized ionic liquid within the pores of the network, where the porous polymer network is asymmetric to mitigate loss of the ionic liquid to a surrounding environment.
Further, it is an object of this disclosure to provide a method for production of a fabricated fiber comprised of an asymmetric polymer network and immobilized ionic liquid utilizing a dry-wet spinning process, in order to avoid separate fabrication steps of fiber spinning and subsequent ionic liquid soaking.
Further, it is an object of this disclosure to provide a method for production of a fabricated fiber comprised of an asymmetric polymer network and immobilized ionic liquid, where the fabricated fiber is suitable for use in the separation of specific chemical species from a mixture.
Further, it is an object of this disclosure to provide a method for production of a fabricated fiber comprised of an asymmetric polymer network and immobilized ionic liquid, where the fabricated fiber is suitable for use in the separation of specific chemical species from a mixture, and to provide a method of facilitating the separation utilizing the fabricated fiber.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.